Seed formation, apomixsis, polyembrony

1. Seed
• Seed described as a fertilised ovule..
• A seed typically consists of • seed coat
• cotyledons
• embryo axis.

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3. Non-albuminous and albuminous seeds.
Mature seeds
Non-albuminous: Albuminous
• Seeds have no residual
endosperm.
• Endosperm completely
consumed during embryo
development..
• (e.g., pea, groundnut).
• Albuminous seeds retain
a part of endosperm.
• Endosperm is not
completely used up
during embryo
development ..
• (e.g., wheat, maize,
barley, castor

4. Perisperm.
• In some seeds such as black pepper and beet, remnants
of nucellus are also persistent.
• This residual, persistent nucellus is the perisperm.

5. Seed
The cotyledons of the embryo are simple structures,
generally thick and swollen due to storage of food reserves
(as in legumes).

6. • Integuments of ovules  Protective seed coats.
• The micropyle remains as a small pore in the seed coat.
• Micropyle facilitates entry of oxygen and water into the
seed during germination.
Castor
seeds
AFTER FERTILIZATOIN PROCESS

7. SEED DORMANCY.
• As the seed matures, its water content is reduced and
seeds become relatively dry (10-15 % moisture by mass).
• The general metabolic activity of the embryo slows
down.

8. SEED DORMANCY.
• The embryo may enter a state of inactivity called
dormancy.
• If favourable conditions are available (adequate
moisture, oxygen and suitable temperature), they
germinate…

9. • Endosperm is persistent in cereals – wheat, rice
and maize.

10. • ovules mature  seeds.
• The ovary  fruit.
• The wall of the ovary  wall of fruit  Pericarp.
• Fleshy fruits: Guava, orange, mango, etc.,
• Dry fruits : groundnut, and mustard, etc.
AFTER FERTILIZATOIN PROCESS

12. True fruits
• Most fruits however develop only from the ovary and
are called true fruits.
• Eg: Mango, Citrus, Orange etc..
• The other floral parts degenerate and fall off.

13. False fruits:
• In a few species such as apple, strawberry, cashew, etc.,
the thalamus also contributes to fruit formation.
The edible part in strawberry is thalamus.
pedicel

14. Achenes
• The strawberry is an aggregate of achenes (the true
fruits which contain the seed).
• Achenes attached in an orderly fashion to the epidermis
of the thalamus …

15. Parthenocarpic fruits
• In a few species fruits develop without fertilisation.
• Banana and grape is one such example.
• Auxins, gibberellins are used to induce parthenocarpy..

16. Seeds offer several advantages to
angiosperms.
• Reproductive processes such as pollination and
fertilisation are independent of water, seed formation is
more dependable.
• Seeds have better adaptive
strategies for dispersal to
new habitats and help the
species to colonise in other
areas..

17. • Seeds have sufficient food reserves, young seedlings are
nourished until they are capable of photosynthesis.
• The hard seed coat provides protection to the young embryo.
• Seeds are products of sexual reproduction, they generate new
genetic combinations leading to variations.

18. Seed is the basis of our
agriculture.
• Dehydration and dormancy of mature seeds are crucial
for storage of seeds..
• Stored seeds used as food throughout the year and also
to raise crop in the next season…

19. How long seeds remain alive after they are
dispersed?
• In a few species the seeds lose viability within a few
months.
• Seeds of a large number of species live for several years.
• Some seeds can remain alive for hundreds of years.

20. The oldest is that of a lupine seeds
• Lupinus arcticus excavated from Arctic Tundra.
• The seed germinated and flowered after an estimated
record of 10,000 years of dormancy.
Archeological digging

21. Date palm seed
• A recent record of 2000 years old viable seed is of the
date palm.
• Phoenix dactylifera (Date palm) discovered during the
archeological excavation at King Herod’s palace near the
dead Sea.

22. Tiny seeds
• Some plants in which fruits contain very small and large
number of seeds.
• Orchid, Orobanche , striga, Ficus (Fig tree) each fruit
contain thousands of tiny seeds.
• Orobanche and Striga are parasitic plants.
Cuscuta

23. Epiphyte
• An epiphyte is an organism that grows on the surface
of a plant and derives its moisture and nutrients
from the air…
e.g., Vanda

24. APOMIXIS
• The process of formation of seeds without fertilisation,
called apomixis. ..
• Eg: Asteraceae and grasses.
• Such seeds is called apomictic seeds….
• Apomixis is a form of asexual reproduction that mimics
(to imitate closely) sexual reproduction..

25. Apomictic seeds
• There are several ways of development of apomictic
seeds.
• In some species, the diploid egg cell is formed without
reduction division and develops into the embryo without
fertilisation.

26. Polyembryony.
• Occurrence of more than one embryo in a seed is
referred to as polyembryony..
• Example : Citrus and Mango..

27. Polyembryony.
• More often, as in many Citrus and Mango varieties some of
the nucellar cells surrounding the embryo sac..
• Nucellar cells start dividing, protrude (extend) into the
embryo sac and develop into the embryos.
Nucellar
cells

28. Importance of apomixis in hybrid
seed industry,
• Hybrid varieties of several of our food and vegetable
crops are being extensively cultivated.

29. Importance of apomixis in hybrid
seed industry,
• The seeds collected from hybrids do not maintain hybrid
characters (Because of segregation).
• Production of hybrid seeds is costly and hence the cost
of hybrid seeds become too expensive for the farmers..

30. • If these hybrids are made into apomicts, there is no
segregation of characters in the hybrid progeny.
• Then the farmers can keep on using the hybrid seeds to
raise new crop year after year.

32. Molecular basis of
inheritance

33. DNA and RNA are Genetic materials
• Deoxyribonucleic acid (DNA) and Ribonucleic acid
(RNA) are the two types of nucleic acids found in
living systems…
• DNA acts as the genetic material in most of the
organisms.
• RNA also acts as a genetic material in some viruses….

34. RNA also acts as a genetic material in some viruses….

35. RNA has additional roles..
• Mostly RNA functions as a messenger RNA (m-RNA).
• RNA functions as adapter (t-RNA/transfer RNA).
• RNA functions as Structural RNA (r-RNA/ Ribosomal RNA).
• In some cases function as a catalytic molecule
(Enzyme/Ribozyme).

36. • Nucleotides  DNA DNA+ Histone proteins 
Nucleosomes Chromatin  Chromosomes Genome

37. Nucleic acids (DNA/RNA) are polymers
nucleotides
• DNA is a long polymer of deoxyribonucleotides.
• RNA is a long polymer of ribonucleotides.

38. THE DNA
• DNA is a long polymer of deoxyribonucleotides.
A nucleotide has three components:
• (ribose in case of RNA, and deoxyribose for DNA),
Nitrogenous
base, Pentose
(or)
Ribose
sugar
Phosphate
group

39. The length of the DNA
• The length of DNA is usually defined as number of
nucleotides..
• or a pair of nucleotide referred to as base pairs present in
it.

40. Single strand DNA
• The length of ss DNA is usually
defined as number of
nucleotides..

41. • The length of ds DNA is
usually defined as
number of nucleotides..

42. A Bacteriophage
known as φ ×174 has
5386 nucleotides
Bacteriophage lambda
has
48502 base pairs (bp)

43. Haploid content of human DNA is
3.3 × 109 bp.
Escherichia coli has 4.6 × 106 bp

44. Structure of Polynucleotide Chain
• DNA is a polynucleotide chain.
• A nucleotide has three components:
1. a nitrogenous base,
2. a pentose sugar (ribose in case of RNA, and
deoxyribose for DNA),
3. A phosphate group.

45. There are two types of nitrogenous bases
• Thymine is present in DNA. Uracil is present in RNA at
the place of Thymine.
Purines:
(Adenine and Guanine):
Pyrimidines
(Cytosine, Uracil and
Thymine).

46. • In DNA
• A=T
• G=C
• In RNA
• A=U
• G=C
• Thymine also called as 5-methyl uracil…

47. Deoxyribose sugar and Ribose sugar

48. N-glycosidic
linkage
Phosphate group is linked to 5' -OH of a
nucleoside through phosphoester
linkage,

49. Uracil Uridine Uridylic acid
Nitogen base +Sugar +Phosphate

50. • Two nucleotides are linked through 3'-5' phosphodiester
linkage to form a dinucleotide.
• More nucleotides can be joined in such a manner to form
a polynucleotide chain..

51. • One end of polymer is called 5’-end contain free
phosphate moiety at 5' -end of ribose sugar.
• The other end of the polymer the ribose has a free 3' -OH
group which is referred to as 3'-end of the polynucleotide
chain.

52. • The backbone in a
polynucleotide
chain is formed
due to sugar and
phosphates.

53. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
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